internal/pipewire: use custom marshaler when available

This reduces special cases. This change also exposes unmarshalled message size on the wire.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/core.go

@@ -76,4 +76,7 @@ type CoreHello struct {
 func (c *CoreHello) MarshalBinary() ([]byte, error) { return MarshalAppend(make([]byte, 0, 24), c) }
 
 // UnmarshalBinary satisfies [encoding.BinaryUnmarshaler] via [Unmarshal].
-func (c *CoreHello) UnmarshalBinary(data []byte) error { return Unmarshal(data, c) }
+func (c *CoreHello) UnmarshalBinary(data []byte) error {
+	_, err := Unmarshal(data, c)
+	return err
+}

internal/pipewire/pod.go

@@ -65,6 +65,13 @@ const (
 	_SPA_TYPE_LAST // not part of ABI
 )
 
+// PODMarshaler is the interface implemented by an object that can
+// marshal itself into PipeWire POD encoding.
+type PODMarshaler interface {
+	// MarshalPOD encodes the receiver into PipeWire POD encoding and returns the result.
+	MarshalPOD() ([]byte, error)
+}
+
 // An UnsupportedTypeError is returned by [Marshal] when attempting
 // to encode an unsupported value type.
 type UnsupportedTypeError struct{ Type reflect.Type }
@@ -86,11 +93,12 @@ func MarshalAppend(data []byte, v any) ([]byte, error) {
 	return marshalValueAppend(data, reflect.ValueOf(v))
 }
 
-// marshalValueAppendRaw implements [MarshalAppend] on [reflect.Value].
-func marshalValueAppend(data []byte, v reflect.Value) ([]byte, error) {
+// appendInner calls f and handles size prefix and padding around the appended data.
+// f must only append to data.
+func appendInner(data []byte, f func(data []byte) ([]byte, error)) ([]byte, error) {
 	data = append(data, make([]byte, 4)...)
 
-	rData, err := marshalValueAppendRaw(data, v)
+	rData, err := f(data)
 	if err != nil {
 		return data, err
 	}
@@ -108,6 +116,18 @@ func marshalValueAppend(data []byte, v reflect.Value) ([]byte, error) {
 	return rData, nil
 }
 
+// marshalValueAppendRaw implements [MarshalAppend] on [reflect.Value].
+func marshalValueAppend(data []byte, v reflect.Value) ([]byte, error) {
+	if v.CanInterface() {
+		if m, ok := v.Interface().(PODMarshaler); ok {
+			extraData, err := m.MarshalPOD()
+			return append(data, extraData...), err
+		}
+	}
+
+	return appendInner(data, func(data []byte) ([]byte, error) { return marshalValueAppendRaw(data, v) })
+}
+
 // marshalValueAppendRaw implements [MarshalAppend] on [reflect.Value] without the size prefix.
 func marshalValueAppendRaw(data []byte, v reflect.Value) ([]byte, error) {
 	switch v.Kind() {
@@ -146,6 +166,15 @@ func marshalValueAppendRaw(data []byte, v reflect.Value) ([]byte, error) {
 	}
 }
 
+// PODUnmarshaler is the interface implemented by an object that can
+// unmarshal a PipeWire POD encoding representation of itself.
+type PODUnmarshaler interface {
+	// UnmarshalPOD must be able to decode the form generated by MarshalPOD.
+	// UnmarshalPOD must copy the data if it wishes to retain the data
+	// after returning.
+	UnmarshalPOD(data []byte) (Word, error)
+}
+
 // An InvalidUnmarshalError describes an invalid argument passed to [Unmarshal].
 // (The argument to [Unmarshal] must be a non-nil pointer.)
 type InvalidUnmarshalError struct{ Type reflect.Type }
@@ -164,12 +193,17 @@ func (e *InvalidUnmarshalError) Error() string {
 // Unmarshal parses the JSON-encoded data and stores the result
 // in the value pointed to by v. If v is nil or not a pointer,
 // Unmarshal returns an [InvalidUnmarshalError].
-func Unmarshal(data []byte, v any) error {
+func Unmarshal(data []byte, v any) (size Word, err error) {
 	rv := reflect.ValueOf(v)
 	if rv.Kind() != reflect.Pointer || rv.IsNil() {
-		return &InvalidUnmarshalError{reflect.TypeOf(v)}
+		return 0, &InvalidUnmarshalError{reflect.TypeOf(v)}
 	}
-	return unmarshalValue(data, rv.Elem(), new(Word))
+	err = unmarshalValue(data, rv.Elem(), &size)
+	// size and type prefix
+	size += 8
+	// padding
+	size += (8 - (size)%8) % 8
+	return
 }
 
 // UnmarshalSetError describes a value that cannot be set during [Unmarshal].
@@ -199,6 +233,22 @@ func (e StringTerminationError) Error() string {
 
 // unmarshalValue implements [Unmarshal] on [reflect.Value].
 func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
+	if !v.CanSet() {
+		return &UnmarshalSetError{v.Type()}
+	}
+
+	if v.CanInterface() {
+		if v.Kind() == reflect.Pointer {
+			v.Set(reflect.New(v.Type().Elem()))
+		}
+
+		if u, ok := v.Interface().(PODUnmarshaler); ok {
+			var err error
+			*sizeP, err = u.UnmarshalPOD(data)
+			return err
+		}
+	}
+
 	switch v.Kind() {
 
 	case reflect.Int32:
@@ -206,9 +256,6 @@ func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
 		if err := unmarshalCheckTypeBounds(&data, SPA_TYPE_Int, sizeP); err != nil {
 			return err
 		}
-		if !v.CanSet() {
-			return &UnmarshalSetError{v.Type()}
-		}
 		v.SetInt(int64(binary.NativeEndian.Uint32(data)))
 		return nil
 
@@ -223,7 +270,7 @@ func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
 				return err
 			}
 			paddingSize := (8 - (fieldWireSize)%8) % 8
-			// already bounds checked by the successful unmarshalValue call
+			// bounds check completed in successful call to unmarshalValue
 			data = data[8+fieldWireSize+paddingSize:]
 		}
 
@@ -233,10 +280,6 @@ func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
 		return nil
 
 	case reflect.Pointer:
-		if !v.CanSet() {
-			return &UnmarshalSetError{v.Type()}
-		}
-
 		if len(data) < 8 {
 			return io.ErrUnexpectedEOF
 		}